Isopropyl N-phenylcarbamate (IPC) and isopropyl(N-(3-chlorophenyl) carbamate (CIPC) are herbicides that cause microtubule disorganization, mitotic arrest and growth inhibition in higher plants, ferns, algae and protozoa. We have shown that concentrations of CIPC that affect plant cells also cause microtubule disassembly in mouse 3T3 fibroblasts. However, this is probably not a primary response to the herbicide. First, CIPC inhibits neither the assembly of purified mammalian brain tubulin nor its colchicine binding activity. Second, microtubule disassembly is preceded by a redistribution of cytoplasmic microfilaments into dense discontinuous bands under the cell membrane. Third, microtubules reassemble after CIPC withdrawal and yet a high proportion of cells develop multipolar mitotic spindles, multiple nuclei and show unequal division of nuclei between daughter cells during cytokinesis. We will further characterize these morphological abnormalities in order to assess the potential genetic risk to people working with the carbamate "herbicides" as well as to determine if the fragmented nuclei may be of value to future genetic mapping and recombinant studies. At the same time, we will concentrate effort into determining if the contractile microfilaments, which influence cell shape, nuclear and cell division and may be involved in mitosis, represent a primary target of CIPC action in animal, and probably plant systems.